David beat me to it, as I was going to say essentially the same
thing. I think what you want is
Macros: {
user: ['\\pmb{#1}',1]
}
so that \user1 will get you a bold 1, and \user2 will get you a bold
2. You might be better off using \boldsymbol rather than \pmb as the
result will generally look better, although there are some symbols
that don't have a bold form in the MathJax fonts, so perhaps that is
why you use \pmb.
Also note that the last entry in your Macros block should NOT have a
comma after it, as that will confuse some versions of IE.
As for \nulldelimiterspace, see
https://groups.google.com/d/topic/mathjax-users/YZah00dYI1Y/discussion
You can add a prefilter to the TeX processing via
<script type="text/x-mathjax-config">
MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () {
MathJax.InputJax.TeX.prefilterHooks.Add(function (data) {
data.math = data.math.replace(/\\kern *-\\nulldelimiterspace/g,"");
});
});
</script>
(place this BEFORE the script that loads MathJax.js itself). This
will remove all the \kern -\nulldelimiterspace references in your TeX
equations before processing them.
Davide